Initial draft of localization docs

Author: aomarks

packages/lit-dev-content/site/guide/libraries/localization.md

@@ -0,0 +1,9 @@
+---
+title: Localization
+eleventyNavigation:
+  key: Localization
+  order: 5
+---
+
+<!-- This file exists only to create a section heading.
+     Its output is deleted by the Eleventy build process. -->